Wide area radio frequency plasma apparatus for processing multiple substrates

ABSTRACT

An antenna array for a radio frequency plasma process chamber including, an array of electrodes, an array of dielectric tubes concentrically disposed about each electrode tube to define a chamber configured to be at atmospheric pressure between an outer surface of each electrode tube and an inner surface of the corresponding dielectric tube, and a hermetic seal between each dielectric tube and the plasma process chamber configured to allow a vacuum or low pressure in the plasma process chamber.

BACKGROUND

The present disclosure relates to a semiconductor apparatus, and moreparticularly, to a wide area radio frequency plasma source suitable forsimultaneously ashing or etching multiple semiconductor substrates.

In ashing and etching applications, the use of radio frequency (RF) ormicrowave power is common. In induction RF plasma reactors, induction byRF occurs by generating a plasma discharge with a metal exciter i.e.,electrode (typically in the form of a coil). Typically, in a capacitivedischarge the wafer holder serves as an opposing electrode, while at thesame time acts as a heating/cooling platen in order to maintain certainsubstrate temperature. The metal exciter is physically removed from theplasma discharge by a dielectric window of some sort, through which thehigh-frequency energy can be coupled. Having the metal exciter separatedfrom the plasma discharge by a dielectric is necessary to prevent theplasma from striking the metal exciter and causing sputtering of metalfrom the exciter, which can deposit on the wafers leading to defects onthe wafers, and substantially shorten exciter lifetime. As used herein,the term “wafer” shall mean any material substrate, including but notlimited to silicon wafers, glass panels, dielectrics, metal films orother semiconductor material.

RF power at 13.56 MHz is predominantly used in plasma reactors becausethis frequency is an ISM (Industry, Scientific, Medical) standardfrequency for which government mandated radiation limits are lessstringent than at non-ISM frequencies, particularly those within thecommunication bands. The substantially universal use of 13.56 MHz isfurther encouraged by the large amount of equipment available at thatfrequency because of this ISM standard. Other ISM standard frequenciesare at 27.12 and 40.68 MHz, which are the second and third orderharmonics of the 13.56 MHz ISM standard frequency.

In the semiconductor industry, throughput is often a very importantissue. With large volumes and low profit margins in the more competitiveareas, incremental improvements in throughput can provide the necessaryedge to compete successfully. In order to reduce manufacturing costs andincrease throughput, it is advantageous to process more than one wafersimultaneously. Not only does this reduce the cost of ownership for theprocess tool, but also, the cost of generating the plasma can beamortized over multiple wafers thereby reducing the production cost perwafer. The difficulty in simultaneously processing multiple wafers in RFreactors is that significant mechanical problems arise. For example,when multiple wafers are processed in the same vacuum chamber and usingone RF exciter, the excitation region is about 70 cm in diameter, whichwould require a very large, thick and heavy dielectric to make thevacuum. For such a three-wafer vacuum chamber, a single quartz piece(dielectric) is projected to be as great as 8 cm thick and weigh greaterthan 90 kilograms (kg), making it very expensive. Moreover, plasmauniformity is generally an issue if one simply increases the areaprocessed in the RF reactor.

Accordingly, there remains a need for improved apparatuses forprocessing multiple wafers simultaneously.

BRIEF SUMMARY

Disclosed herein are plasma generating components, apparatuses, andmethods for generating a plasma. In one embodiment, an antenna array fora radio frequency plasma process chamber comprises an array of metalelectrodes; a dielectric tube concentrically disposed about each metalelectrode of the array of metal electrodes; and a hermetic seal betweeneach dielectric tube and the plasma process chamber.

In another embodiment, a plasma generating apparatus for simultaneouslyprocessing one or more substrates within a single vacuum process chambercomprises a gas source; a vacuum process chamber adapted to process theone or more substrates simultaneously, the process chamber having a topwall, a bottom wall and sidewalls extending therebetween, the top wallincluding one or more openings in fluid communication with the gassource, an antenna array intermediate the one or more openings and awafer pedestal, wherein the antenna array comprises a plurality ofhermetically sealed and serially connected conductor segments, each oneof the conductor segments comprising an electrode and a dielectricmaterial concentrically disposed about the electrode; and a power sourceelectrically coupled to the antenna array.

A method of plasma processing multiple wafers in a process chamberincludes, introducing a source gas to the process chamber, wherein theprocess chamber comprises a top wall, a bottom wall and sidewallsextending therebetween, the top wall including one or more openings influid communication with the gas source, a wafer pedestal configured tocontrol the temperature of a wafer, an antenna array intermediate theone or more openings and the wafer pedestal comprising a plurality ofhermetically sealed and serially connected conductor segments, each oneof the conductor segments comprising an electrode and a dielectricmaterial concentrically disposed about the electrode, and a power sourceelectrically coupled to the antenna array, changing an operatingpressure within the process chamber and maintaining atmospheric pressurewithin the conductor segment, and generating a plasma within the processchamber by flowing a current through the conductor segments of theantenna array with a power source.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Refer now to the figures, which are exemplary embodiments, and whereinlike elements are numbered alike:

FIG. 1 shows a partial perspective view illustrating a wide area RFplasma reactor apparatus capable of processing up to four waferssimultaneously;

FIG. 2 illustrates a partial perspective exploded view illustrating thearea RF plasma reactor apparatus of FIG. 1;

FIG. 3 is a cross-sectional view of an exemplary conductor segment;

FIG. 4 shows a perspective view of the antenna system, which generatesplasma in the process chamber;

FIG. 5 illustrates a sectional view of a cooling tube connection forconnecting the conductor segments; and

FIG. 6 illustrates a top-down view of a RF plasma reactor apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 generally illustrate a wide area RF plasma reactorapparatus 10 suitable for use in ashing or etching applications wheremultiple wafers can be processed simultaneously. Briefly stated, it hasbeen discovered that by placing high frequency excitation elementsinside the vacuum process chamber (i.e., inside the plasma), with eachelement contained within a dielectric tube, a wide area plasma sourcecan be created without the need for a large and expensive dielectricwindow or bell jar, such as noted in the background section. As such,plasma is prevented from contacting the exciter elements. In oneembodiment, the vacuum connection for the wide surface area needed toprocess four wafers at a time, as an example, can be made using a metallid less than 2 centimeters (cm) thick. Moreover, the excitationelements, collectively referred to as the antenna system, can besubdivided into multiple straight tubes depending on the desired plasmaarea for a given application. A plasma generating apparatus designed inthis manner provides lightweight, low-cost flexibility over the standardplasma generating apparatus.

Referring now to FIGS. 1 and 2, the wide area RF plasma reactorapparatus 10 generally comprises a process chamber 12, a power source14, and an exhaust assembly component 16. Although the apparatus 10 inFIG. 1 has a square shape, other suitable shapes will be apparent tothose skilled in the art in view of this disclosure and the desiredplasma surface area. Also, the terms “a”, “an”, and “the” do not denotea limitation of quantity, but rather denote the presence of at least oneof the referenced item.

The process chamber 12 generally has a top wall 18, a bottom portion 20,and sidewalls 22 extending therebetween. The top wall further comprisesat least one gas input flanges (openings) 24, and in the present examplethere are four flanges. The process chamber 12 further comprises a waferpedestal 26 within an interior of the process chamber. The waferpedestals 26 function as a temperature control mechanism for the wafer,whereby a heated platen provides heat to the wafer, or a cooled platenremoves heat from the wafer. In one embodiment, the number of waferpedestals 26 generally corresponds to the number of gas flanges.Likewise, the diameter of the gas flanges is about the same as orgreater than the diameter of a wafer pedestal 26.

In the embodiment described above, each gas input flange 24 ispositioned such that when the process chamber 12 is sealed, each flange24 is coaxial to the corresponding wafer pedestal 26. It should be clearto those practiced in the art that the number of gas input flanges neednot be coaxial to the pedestal, and need not be equal to the number ofpedestals in the chamber. A gas source (not shown) may be disposed influid communication with gas input flange 24. Suitable gases forgenerating plasma are well known to those skilled in the art of bothetching and ashing, which include, but are not intended to be limitedto, oxygen or oxygen containing gases, fluorine containing gases,hydrogen or hydrogen containing gases, helium, argon, neon, other inertgases, hydrocarbons, and combinations comprising one or more of theforegoing gases. The wafer substrate pedestal 26 can be any suitablesupport generally known in the art such as, heated wafer chucks, liftpins, and the like.

The process chamber 12 further includes an antenna system comprising anplanar array 28 of single antenna conductors 30 coupled together and inelectrical communication with the power source 14 and with discreteelectric components. Each conductor 30 is substantially parallel to anadjacent conductor. The antenna array 28 in the present example extendsfrom one sidewall to an opposing sidewall to form a grating and ispositioned intermediate the gas flanges 24 and the underlying waferpedestal 26. As will be discussed in greater detail below, the antennaarray 28 provides excitation energy for plasma generation of gasesflowing through the gas flanges 24 within the process chamber 12.

Additional openings 19 may also be disposed in the process chamber 12for purposes generally known in the art such as, for example, a massspectrometer inlet for analyzing gaseous species evolved duringprocessing, endpoint detection, and the like. Moreover, the processchamber 12 may further include additional features depending on theapplication. For example, in ashing applications, a quartz window may beinstalled and a UV light source may be placed in proximity to the wafer.Such a non-columnar light source may have a wavelength similar to UVexcited lasers, which have been shown to enhance photoresist removal inbulk strip applications, and as such, could be used in parallel with theplasma generated reactive gases. Moreover, pre- and post-photoresiststrip exposure to the light source could also provide residue removaland implanted resist removal advantages. Overhead RF sources, opticalports, gas analyzers, additional light sources, and the like could alsobe used either independently, or in combination, with the processchamber 12 providing an extremely flexible process platform. Otheropenings include one or more slit valves (not shown) disposed in thesidewall 22 for inserting and removing the substrates from the processchamber 12.

The process chamber 12 also includes an exhaust opening (not shown)disposed in the bottom wall 20 so as to provide an axial fluid flow inthe chamber 12. An inlet of the exhaust conduit 32 is fluidly attachedto the exhaust opening below each wafer chuck in the process chamber 12.It is to be understood that the exhaust conduit 32 has been simplifiedto illustrate only those components that are relevant to anunderstanding of the present disclosure. Those of ordinary skill in theart will recognize that other components may be required to produce anoperational plasma generating apparatus 10. However, because suchcomponents are well known in the art, and because they do not furtheraid in the understanding of the present disclosure, a discussion of suchcomponents is not provided.

Depending on the desired process (i.e., etching, ashing), operatingpressures within the process chamber 12 are preferably about 1 millitorrto about 3 torr, with about 200 millitorr to about 2 torr morepreferred, and with about 500 millitorr to about 1.5 torr even morepreferred. These operating pressures in the chamber are achieved usingadequate process gas flows through the gas source and by using athrottle, or butterfly valve in fluid contact with the exhaust conduit24 and exhaust opening. The power is in a range of less than about 100watts up to a few thousand watts, at a frequency of about 0.5 megahertz(MHz) to 30 MHz.

Turning now to FIG. 3, a single conductor segment 30 of the antennaarray 28 is illustrated. The conductor segment 30 comprises a metalelectrode 34 in the form of a tube. In one embodiment, the metalelectrode 34 may be a solid rod. The electrode tube may further beselected to have an outer diameter up to 0.25 inches. Surrounding theelectrode 34 is a dielectric material 36. The dielectric material 36 isin the form of a tube and encases the electrode 34 to define a chamberbetween an outer surface of the electrode tube 34 and an inner surfaceof the dielectric tube 36. Fluid, such as air or low loss dielectricfluid, under positive pressure, can be fed through this chamber betweenthe two tubes for system cooling purposes.

A washer 35 is disposed intermediate the electrode 34 and the dielectrictube 36. The washer 35 may be formed of a ceramic, a fluoropolymer, andthe like. The washer 35 has an inner diameter just larger than an outerdiameter of the electrode 34, yet smaller than an inner diameter of thedielectric tube 36. The washer 35 advantageously keeps the electrode 34floating to inhibit the cool electrode 34 from contacting the hotdielectric tube 36 and prevent thermal shock. A hermetic seal 40 placedat each end of the dielectric tube 36 seals the dielectric tube 36 toselected walls 22 of the process chamber 12. Consequently, the processchamber 12 can be maintained at a desired vacuum pressure while theelectrode 34 within the dielectric tube 36 remains at atmosphericpressure, even though the antenna array 28 is exposed to the vacuumpressure. The electrode 34 is preferably made of copper, but can be anymetal commonly used as suitable electrodes for plasma generation such asaluminum, brass, copper-beryllium, etc. The dielectric tube 36 ispreferably made of quartz. However, other dielectric materials such assapphire or ceramic materials can also be used. Further, the electrode34 may also be made of a dielectric material, such as those disclosedabove, which has an outer surface coated with a conducting metal, suchas aluminum, silver, copper, and the like. In another optionalembodiment, the electrode 34 can be the dielectric tube 36 with anelectrically conducting coating on the inner surface of the dielectrictube. Additionally, cooling fluid, such as water, can be passed throughthe electrode tube 34 to provide effective cooling of the conductorsegments during plasma generation.

Referring now to FIG. 4, a perspective view of the antenna array 28 isillustrated. In the present example, the antenna array 28 is comprisedof 16 separate conductor segments 30. In this example, each conductorsegment 30 is spaced 5 cm apart and the antenna array 28 illustrates apossible design for covering up to four standard 300 millimeter (mm)wafers in one chamber. It should be understood, however, that any number“N” of such conductor segments 30 may be employed and such variationsare contemplated by the present disclosure, wherein “N” is an integergreater than 1. Similar antenna systems with varying numbers ofconductor segments and alternate spacing may be suitable in view of theparticular process application and the desired plasma surface area andwill be apparent to those skilled in the art. The antenna array 28 isfed with energy by the power source 14 (shown in FIG. 1) which travelsthrough each electrode tube 34 of each conductor segment 30. Althoughreference is made to inductively coupling the gas mixture with RF powerto form the plasma, other means could be employed in an effective mannersuch as by capacitive excitation or a combination thereof. Additionally,other frequencies in the ISM band may be used to excite the plasmadepending on the desired application. Frequencies outside of the ISMband may also be employed, given adequate shielding of any strayradiation such that radiation emission levels from the tool are withinFCC regulations. A variable frequency system, such as, but not limitedto, the one described in U.S. Pat. No. 6,305,316, incorporated herein byreference in its entirety, may also be used as a power source. In such avariable frequency system the load reactance is part of the tank circuitthat determines the frequency of operation, and as the load changes,e.g., during the removal of photoresist from the substrate, thefrequency changes to adapt to the load. One advantage of this techniqueis that by monitoring the frequency change one can very effectively andreliably determine end-of-process on the wafers without the addedexpense and complexity of optically monitoring the plasma glow forsurface reaction products.

The electrically continuous inductive antenna array will provideexcitation for plasma generation within process chamber 12.

In one embodiment, each conductor segment 30 can be connected with theadjacent conductor segment in the array 28 with a cooling tube 42. Thecooling tube 42 is formed of an RF compatible material and allows acooling fluid to pass through the entire antenna array 28, providingeffective cooling of the system during plasma generation. The coolingtubes 42 are connected to each conductor segment with dielectricfittings 44. The fittings 44 and cooling tubes 42 are preferably made ofTeflon or any other compatible dielectric. FIG. 5 illustrates thecooling tube connection.

Because the path length of the electrical current supplied by powersource 14 becomes large as the number of conductor segments 30 inantenna array 28 increases, a large voltage difference across theantenna array 28 may be generated. Higher voltage portions of theantenna array 28 will couple more energy capacitively to the plasma thanlower voltage portions, leading to plasma non-uniformities. In order toreduce the voltage difference problem, the conductor segments in thearray 28 are serially coupled together through capacitors 31, and two ofthe conductor segments are coupled externally to the power source 14.

The antenna array 28 of the present disclosure operates in the followingmanner. A time dependent current is generated in the conductor segments30 via the power source 14. The time-varying current produces a magneticfield that surrounds the conductive segments in accordance withFaraday's law. Because the current is time-varying, the producedmagnetic field is a time-varying field. In accordance with Maxwell'sequations, the time-varying magnetic field induces a time-varyingelectric field normal thereto; wherein the time-varying electric fieldextends along a direction of the conductor segments 30 and decays as thefield extends away therefrom. This time-varying electric field isreferred to as the inductive electric field component since it isinduced from the time-varying magnetic field.

The time-varying inductive electric field accelerates charged particlessuch as free electrons in the chamber near the conductor segments 30.Further, the conductor segments are configured such that the velocity ofthe accelerated charged particles is sufficient so that the chargedparticles move through the region associated with a conductor in a timethat is short compared to the period (T) of the time-varying current.Consequently, the charged particles see a substantially steady field asit travels along the conductor segment 30. Therefore the time-varyingelectric field “heats” the charged particles that then have sufficientenergy to ionize the source gas atoms within the process chamber 12 uponcollision therewith. The ionizing collisions operate to generate theplasma and such plasma generation is substantially symmetric inaccordance with the configuration of the antenna array 28.

The antenna array 28 has a voltage across each conductor segment thatspatially varies along a length of the electrode tube 34. Consequently,the varying charge distribution along a conductor segment 30 produces anelectrostatic field that extends from the conductor segments outwardly,and the strength of the field varies spatially along the length of theelectrode tube 34. This electrostatic field component is referred to asthe capacitive field component. Because this field is not uniform, thecontribution of this field component to plasma generation isnon-uniform, and thus it is desirable for this electric field componentto be reduced as much as possible.

A faraday shield may be used to block or minimize the capacitive fieldcomponent. In such a solution, the faraday shield is placed between theconductors and the plasma. Such a solution, however, increases circuitlosses and is practically difficult to configure. The antenna system 28of the present disclosure overcomes the disadvantages of the prior artand provides a structure that substantially reduces the capacitive fieldcontribution of the system without use of a faraday shield, as will befurther appreciated below.

The antenna array 28 divides what conventionally was a single conductorinto “N” conductor segments 30 (e.g., N=16 as illustrated in FIG. 4),wherein each conductor segment 30 is direct current isolated from oneanother, but in series via capacitors. Such an arrangement reduces thepeak capacitive electric field component by a factor of N. Preferablythe values of L (associated with the conductors 30), C (the capacitors),and f (the frequency of the signal from the power source 14) areselected so that the magnitude of the reactance of the inductivecomponent (27πfL) is equal to the magnitude of the reactance of thecapacitive component (½πfC). In the above manner, a resonant circuitexists, in which the voltage drop across each inductive element L isequal and opposite to the voltage drop across each capacitive element C.Thus, the maximum voltage drop is reduced by a factor of “N” compared tothe case of N inductive elements in series without capacitive elements.Also, although some variation still occurs along a length of a conductorsegment 30, it is N times smaller than a conventional arrangement, andas illustrated in FIG. 4, such variation is itself symmetric due to thearrangement of the conductor segments.

The antenna array 28 thus operates to generate plasma within the processchamber 12, wherein the generated plasma is symmetric. The symmetry ofthe plasma advantageously helps to provide a spatially uniform currentat the wafers. Optionally, an electromechanically controlled mechanismcan be used to selectively turn off and/or turn on sections of theantenna array 28 to allow plasma processing in only certain portions ofthe process chamber.

FIG. 6 shows another exemplary embodiment of a RF plasma reactorapparatus 60. The plasma reactor 60 generally comprises a processchamber 62. The process chamber 62 has a gas input flange 64, but mayoptionally have more than one gas input flange. Likewise, the processchamber 62 further comprises a wafer pedestal 66, but may optionallyhave more than one pedestal. The process chamber further includes aplanar antenna array 68 composed of a single antenna conductor segments30 in sliding engagement with the side wall of the process chamber.Although three discrete conductors are shown in FIG. 6, it is to beunderstood that any number of conductor segments can be used dependingon various factors, such as process chamber size, number of wafers forsimultaneous processing, plasma region, and the like. While theconductors 30 are similar to those described above and shown in FIGS.3-5, the conductors of FIG. 6 include arcuate portions rather than thelinear rods as shown in FIGS. 3-5. It is to be understood, however, thatthe conductor segments 30 are not limited to the specific shapes shown.For example, if the dielectric material is coated on the outer surfaceof the electrode to form the conductor segment 30, then the conductorcould have any shape. If the dielectric material is a tubeconcentrically disposed about the electrode and is used to form theconductor segment 30, then physical constraints in disposing theelectrode within the tube may limit the choice of shapes, such as wouldbe appreciated by those skilled in the art.

The plasma reactor apparatus 60 advantageously allows for an individualconductor segment 30 to be slideably adjusted through the sidewall inthe process chamber 62. This planar adjustment can beneficially be donewithout touching the other conductor segments 30 of the planar array 68.Moreover, it is possible for the adjustment to be done dynamicallyduring processing. By repositioning, adding, or removing individualconductor segments 30 in the plasma reactor apparatus, plasma generationregions within the process chamber 62 can be dynamically changed toaffect photoresist removal, etching, and plasma uniformity.

Unless otherwise specified, the materials for fabricating the variouscomponents include metals, ceramics, glasses, quartz, polymers,composite materials, and combinations comprising at least one of theforegoing materials. For example, suitable metals include anodizedaluminum, and/or stainless steel. Suitable ceramic materials includesilicon carbide, or aluminum oxide (e.g., single crystal orpolycrystalline).

Advantageously, as mentioned above, the components and systems of plasmageneration disclosed herein may allow for a reduction in production andequipment cost and increased operating life over existing plasmaprocessing systems. More particularly, the wide area plasma generationthat is capable due to the conductive element being disposed within theplasma vacuum chamber rather than outside over a cumbersome andexpensive dielectric window, makes processing multiple wafer substratessimultaneously more cost effective than existing processing apparatuses.Additionally, the antenna system disclosed above generates uniformplasma capable of ashing or etching multiple wafers at the same time,thereby increasing throughput and decreasing production cost per wafer,all without sacrificing plasma process quality.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A plasma generating apparatus for processing one or a plurality ofsubstrates within a single vacuum process chamber, the apparatuscomprising: a gas source; a vacuum process chamber adapted to processthe one or a plurality of substrates, the process chamber having a topwall, a bottom wall and sidewalls extending therebetween, the top wallincluding one or more openings in fluid communication with the gassource and wherein the vacuum process chamber includes a plurality ofwafer pedestals; an antenna array intermediate the one or more openingsand the plurality of wafer pedestals, wherein the antenna arraycomprises a plurality of conductor segments serially coupled togetherthrough capacitors, each one of the conductor segments comprising afloating electrode and a dielectric tube concentrically disposed aboutthe floating electrode, wherein the floating electrode is free fromcontact with the dielectric tube and the electrode floats in a washerdisposed intermediate the electrode and the dielectric tube, and whereinthe dielectric tube is hermetically sealed at each end against thesidewalls of the process chamber such that the vacuum process chambercan be evacuated during operation thereof while the floating electrodeis at atmospheric pressure; a power source in electrical communicationwith two of the conductor segments.
 2. The plasma generating apparatusof claim 1, wherein the plurality of conductor segments are comprised ofindividual conductor segments, wherein at least one of the individualconductor segments is slideably disposed in the sidewall, such thatmovement of the at least one individual conductor segment changes aplanar position.
 3. The plasma generating apparatus of claim 1, whereinthe electrode is a tube.
 4. The plasma generating apparatus of claim 1,wherein each one of the conductor segments is fluidly and seriallycoupled to a cooling tube.
 5. The plasma generating apparatus of claim1, wherein the power source is a radio frequency power supply.
 6. Theplasma generating apparatus of claim 1, wherein the vacuum processchamber has an exhaust aperture disposed in the bottom wall.
 7. Theplasma generating apparatus of claim 1, wherein the antenna array issubstantially planar.
 8. The plasma generating apparatus of claim 1,wherein each one of the conductor segments are equidistantly spacedapart from one another.
 9. The plasma generating apparatus of claim 1,wherein the electrode is formed of a copper metal and the dielectrictube is formed of a quartz metal.
 10. The plasma generating apparatus ofclaim 1, wherein each conductor segment is direct current isolated fromone another and disposed in series via capacitors such that eachconductor segment provides a resonant circuit, with a resonant frequencysubstantially the same as an operating frequency.
 11. The plasmagenerating apparatus of claim 1, further comprising anelectromechanically controlled mechanism configured to selectively turnoff and turn on sections of the antenna array.